Provides the original 1901 French edition and some English translations for borrowing or downloading. Original French Edition (1901) English Translation (1938 Thesis) Open Library:

The treatise systematically dissects the components of fugue writing:

One of the most technically challenging aspects of fugue writing is the tonal answer. Gedalge provides clear rules for modifying the subject in the dominant key to maintain tonal coherence. His explanations of "mutations" (intervallic adjustments in the answer) are considered some of the clearest in theoretical literature.

If you're looking for André Gedalge's Traité de la fugue (Treatise on the Fugue), you're searching for one of the most respected academic resources on music composition.